It was ordered to give €3,000 to charity to avoid a criminal conviction.
The company is being prosecuted by the Environmental Protection Agency (EPA) over a delay in completing a €1m project to deal with a water quality problem in Carraroe.
Earlier this year, residents in the Connemara village received boil water notices.
The prosecution was listed before Judge John O’Neill yesterday at Dublin District Court.
Irish Water pleaded guilty to charges under the EU (drinking water) regulations that, between December 1, 2015, and January 8, 2016, it failed to comply with a directive issued by the EPA to implement an action programme approved by the agency for the improvement of the quality of water for human consumption from Carraroe public water supply, to react to parametric values specified for trihalomethanes (THMs).
THMs are a group of organic chemicals formed when chlorine is used to disinfect water and reacts with naturally occurring organic matter in raw water.
An action plan had been approved by the EPA on September 9, 2014, but the required work was not completed by Irish Water until this year.
Maeve Larkin, prosecuting, said the offence can carry a class A fine of €5,000.
Darragh Page, a senior inspector with the EPA, told the court that for drinking water to be safe, there should be no more than 100mg of THMs per litre. Tests indicated the levels in the water in Carraroe were in excess.
There had been an elevated level for several years until recently, he said.
He agreed with Ms Larkin that the solution was to pre-treat the water using filtration to take out colour or organic matter before using chlorine so by-product THMs are not formed.
An action plan was agreed with Irish Water but it was not fully complied with until this year, the court heard. In January, five weeks after the deadline, Mr Page visited the treatment plant and saw it had been unchanged and the level of THMs still exceeded the standards expected.
The court heard that Irish Water had no prior criminal convictions.
Judge O’Neill said it was not right that Irish Water took for granted that there would not be planning permission difficulties.
However, he agreed to order the company to make a charitable contribution rather than imposing a conviction. He ordered that €3,000 must go to local charity Áras an Dara.
Adjourning the case, Judge O’Neill said if the money is donated by September 30, he would apply the Probation Act, which would spare Irish Water a conviction.
“I will keep my options open if that is not done,” said the judge.